The Chargers have not been particularly effective at stopping running backs. That could spell trouble Sunday in New England, where they’ll have to contend with four of them. Tom Brady is the engine that drives the Patriots’ potent offense, a superstar quarterback who even at 40 years old shows few signs of decline. But this offense has wheels too. There’s Dion Lewis, who in January scored on a run, a catch, and a kickoff return in the same playoff game. There’s James White, the third-down weapon who totaled three touchdowns and 139 yards from scrimmage in Super Bowl LI. Mike Gillislee, who punched in three touchdowns in the season opener. Rex Burkhead, back in the rotation after injuring his ribs last month. All four have started at least one game this season. None has started more than two. “It’s very challenging because they all have different skill sets,” said Coach Anthony Lynn, whose Chargers have allowed 984 rushing yards. “Some of those guys are better out of the backfield as receivers. They’re all good runners. And when they get the ball in their hands they make plays.” Added defensive coordinator Gus Bradley: “We have to be really aware of that. We spent a lot of time this week, just looking at the styles of runs.” Lewis has been New England’s back of choice in the past two weeks, running 24 times during that span for 128 yards and a score. Still, the backfield pecking order can change on any given drive. Gillislee had double-digit carries in every game this season — until he lost a fumble in Week 6. That prompted Coach Bill Belichick to sit him for 34 straight snaps, opening the door for Lewis to pull ahead.